The invention generally relates to the connection of emergency sessions such as emergency calls in abnormal cases, and more specifically refers to bearer establishment for emergency sessions in abnormal situations.
Several problems may arise in ensuring a proper establishment of emergency sessions.
In 3GPP (Third Generation Partnership Program), it has been proposed that an MS (Mobile Station) with USIM (User Subscription Identity Module) performs Attach and signalling PDP context activation. When, at some point, the user of the MS intends to make an emergency session, the MS signals at PDP context activation that the PDP context will be used for an emergency session. In detail, an emergency session can be indicated by e.g. using the Source Statistics Descriptor parameter. The MS is responsible for properly setting the Source Statistics Descriptor parameter so as to indicate that the PDP context will be used for an emergency session.
Emergency sessions and PDP contexts used for them are of highest priority. The network is adapted to guarantee the establishment of an emergency session if it gets an indication and a confirmation classifying the session request to be an emergency session request.
The MS may however not always be able to set the Source Statistics Descriptor parameter. As an example, some MSs are adapted to check and compare a dialed number with stored emergency numbers and to set the Source Statistics Descriptor parameter representing an emergency session when detecting an emergency number. When, however, an actually dialed emergency number is not known by the MS, i.e. not configured to the MS, the MS will not set the Source Statistics Descriptor parameter so as to represent an emergency session.
Even without properly setting the Source Statistics Descriptor parameter, the PDP context activation for an emergency session should proceed. Normally, the serving node such as SGSN (Serving GPRS Support Node) is the first to make the decision whether or not to proceed with the PDP context activation.
However, in case of prepaid services, e.g. CAMEL based prepaid services, the serving node such as SGSN may communicate with a control means such as SCP (Service Control Point) at Attach and at PDP context activation. The control means decides about the continuation. If the prepaid account is empty, the control means normally does not let the user Attach or activate a PDP context. Such a case may also arise when the money in a prepaid account is divided, e.g., between the PDP contexts of a subscriber. The MS may be able to perform Attach and signalling PDP context activation, but there may be no money left in the prepaid account when the MS tries to activate a PDP context for an emergency session. In case of prepaid services, e.g. CAMEL based prepaid services, when establishing a session, another serving node such as CSCF (Call State Control Function) may communicate with a control means such as SCP (Service Control Point). The control means decides about the continuation. If the prepaid account is empty, the control means normally does not let the user establish the session. This is the IMS part. CSCF may communicate with SCP.
Further, there may be a case that the network does not let the MS enter the network. The MS may not be allowed to roam to the network due to subscription based restrictions, or the subscription information of the MS may be faulty, or the network considers the MS to be misbehaving, e.g., if the subscriber has not paid bills. The network may in such cases not allow the MS to perform Attach, i.e. may reject the Attach request. In case of an emergency session, the network should, however, let the Attach, the signalling PDP context activation and the emergency session proceed.